The invention relates to mixed oxide catalysts for the catalytic gas phase oxidation of olefins or methylated aromatics, to processes for preparing the catalysts and to the conversion to aldehydes and carboxylic acids with air or oxygen in the presence of inert gases in different quantitative ratios, at elevated temperatures and pressures.
In particular, the catalyst may be used to implement the highly exothermic reaction of propene to give acrolein and acrylic acid, or isobutene to give methacrolein and methacrylic acid. The highly exothermic reaction of the olefin over heterogeneous catalysts with an oxygen-containing gas leads, in addition to the desired acrolein and acrylic acid product, to a series of by-products, for example to the formation of CO2, CO, acetaldehyde or acetic acid.
It is known that the type of chemical composition of the mixed oxide (phase formation and formation of reaction sites) and also the type of physical structure (for example porosity, surface size, shape of the catalyst), the type of heat removal, can greatly influence the ability to form product (selectivity) and the productivity (space-time yield). In the case of the olefin oxidation, the catalyst used is generally mixed oxides which have a complex chemical and physical structure. A multitude of publications describes mixed oxides which are capable of being used as catalysts for the preparation of acrolein and acrylic acid from propene. These catalysts consist generally of molybdenum, vanadium and/or tungsten. Generally at least one of the elements bismuth, antimony, vanadium, tellurium, tin, iron, cobalt, nickel and/or copper is added to these base components.
The number of publications on the heterogeneously catalysed gas phase oxidation of olefins to acrolein and acrylic acid is numerous since the first development GB 821999 (1958) for Standard Oil Inc. In spite of the long development time, it is still a demanding task to improve the performance of the catalyst, such as product yield, activity and lifetime. On this subject, the literature claims various techniques for preparation, and also formulations of the catalyst. By way of example, the most recent developments are explained here:
US 2005159621 describes a catalyst consisting of the base elements Mo, Bi, Fe, Cs. In addition, it is shown, as in the examples by the conversion of isobutene to methacrolein or propene to acrolein, that the highly toxic antimony is also required for this catalyst.
For the preparation of the catalyst, WO 2005/035115 utilizes the following manufacturing steps: preparation of a suspension which comprises the metal components, drying of the suspension, comminution of the dried material, mixing of the material with a sublimable substance, especially urea for pore generation, which is removed in the calcination. However, the removal of organic additives in the calcination harbours the risk of explosion; controlled removal of the organics is often impossible even with inert gas dilution. It is therefore doubtful whether such a process can be implemented on the production scale.
DE 103 53 954 utilizes catalysts of the Mo12WbCocFedBieSifKgOx type as annular unsupported catalysts in which the task of reducing the maximum temperature increase at high propene loading and hence of increasing the product selectivity is claimed. In the case of single throughput, the propene conversion is said to be greater than or equal to 90 mol % and hence the associated acrolein formation greater than or equal to 80 mol %. In the examples, yields of not more than 83.8% are shown.
WO 2005/063673 describes, for example, a method for diluting the catalyst with an inert material, in order to reduce the heat formation in the reaction zone and hence increase the product yield, by virtue of the avoidance of too high a hot spot reducing the total oxidation of the products. In spite of the temperature modulation of the reaction by inerts, the process described achieves only a total yield of acrolein and acrylic acid of not more than 91.22%. However, it would be more advisable to improve the yield of the catalyst directly. This would allow time-consuming and cost-intensive multizone filling of the reactor to be avoided.